This invention relates to a steering wheel armature coated with a thermoplastic foamed material.
Steering wheels in general comprise a metal armature that is enclosed in an appropriate covering material such as wood, elastomeric substance, or a combination of the two. With the development of technology of attaching the steering wheel with the airbag module juxtaposed to the steering column (i.e. U.S. Pat. No. 5,897,132 to Papandreou and U.S. Pat. No. 5,692,769 to Scharboneau et al.), there became an impetus for developing a single material composite steering wheel/airbag cover. The difficulty with this feat is the physical properties required of a material for the airbag cover are considerably different from those required of a material in the rim of a steering wheel. The obstacle of producing an integral steering wheel (rim, airbag cover, and spokes are molded from same material) has partially been overcome by utilizing reaction injection molded (RIM) polyurethane. This process produces a final surface that has very good wear properties while being compliant to the touch and relatively low in mass. These properties are highly desirable for a steering wheel rim and, in fact many standard (non-integrated) steering wheels are produced using this process. While this process has also been used extensively for airbag covers in the past, the use of RIM urethane is not nearly as desirable for this application due to the tendency of urethane to fragment during airbag deployment. U.S. Pat. No. 5,692,769 teaches that to overcome this drawback a tough, non-fragmenting material such as a substrate or a scrim is added beneath the airbag cover. Thus, it is inherent in this step that a substrate must be produced prior to the final urethane molding phase and must be introduced into the polyurethane molding tool, by some method, prior to introduction of the polyurethane. This step consumes time during the manufacturing process and adds complexity and cost to the molding due to the necessity of adding attachment features to secure the substrate during the RIM process.
The RIM polyurethane process has additional disadvantages. To promote proper filling of the mold cavity many air vents are employed and the tool is closed in such a way that some additional venting occurs at the meeting point of the two halves of the mold. Invariably some polyurethane material fills the vent areas during the molding process with the result that the molded part must later be trimmed to remove the excess material. During the trimming process it is easy to inadvertently damage the final surface of the product rendering it unacceptable for use.
An additional concern with the reaction injection molding process is the duration of the cycle. This cycle is typically two to three times longer in duration than the cycle associated with a standard injection molding process for a similar part.
To overcome the numerous disadvantages stated above it would be desirable to produce the final surface of the integrated steering wheel using an alternate process. The chief alternative used today to produce steering wheels and airbag covers is standard injection molding. Typically, automotive steering wheels are fabricated using thermoplastic polyvinyl chloride (PVC) based resins while airbag covers are fabricated from high-performance engineered resins known generally as thermoplastic elastomers. These two materials, as used in the subject applications, have very different physical properties. PVC is formulated for good wear characteristics and a targeted compressibility or xe2x80x9cfeel.xe2x80x9d However, steering wheel rims made of PVC with plasticizers may encounter adhesion problems with paints or outside covers on the rim due to the migration of the plasticizers to the surface. The thermoplastic elastomers are chosen for chiefly their strength across a large temperature range and exhibit very little compliance. It is generally recognized that improving the compliance characteristics of a thermoplastic produces an adverse effect on strength and vice versa.
In order to produce an integrated steering wheel in a single-shot injection molding process a thermoplastic material must be found that will satisfy the dissimilar performance requirements of the steering wheel rim and airbag cover portions.
In a first embodiment of this invention, a single thermoplastic foam material is injected into a steering wheel mold to produce an integral steering wheel; the composite steering wheel/airbag cover has a continuous external surface molded from a single thermoplastic material. The rim portion and the spoke portions act as support structures for the single thermoplastic foam material.
The rim of the steering wheel has good wear characteristics and a targeted compressibility or xe2x80x9cfeelxe2x80x9d while the airbag cover has the sought-after strength across a large temperature range and exhibits very little compressibility. The airbag cover has the necessary tear strength so that a reinforcing member or substrate does not have to be added underneath the airbag cover.
In a second embodiment of this invention, a cushion and outside wrap, such as leather, cover a steering wheel rim made of thermoplastic foam material. The thermoplastic foam material with a non-olefinic foam carrier, such as polyvinyl acetate (PVA), does not degrade the adhesive used to attach a cushion and/or an outside wrap covering the steering wheel rim. The configuration of the wrap and cushion result in a steering wheel with the required feel and effective adhesion to the steering wheel rim.